This invention relates to trip control devices for monitoring the voltage applied on the circuit breakers and to trip the circuit breakers and thereby to protect the voltage source and the loads. More particularly, this invention relates to such trip control devices which render the circuit breakers incapable of being made and closed when the controlled voltage such as the source voltage exhibit abnormal levels due to overvoltage.
FIG. 3 is a circuit diagram showing a conventional trip control device, which is disclosed, for example, in Japanese Utility Model Publication (Kokoku) No. 58-18441. When the source voltage becomes less than a predetermined level, the electromagnetic coil is de-energized to effect the tripping control of the circuit breaker. The AC voltage supplied from a controlled power source 1 such as a generator is rectified by a full-wave rectifier 2 consisting of a diode bridge circuit. The negative terminal of the full-wave rectifier 2 is grounded, while the positive terminal thereof supplies the controlled voltage V.sub.0 to the respective circuit components.
A PNP type transistor 3 having an emitter coupled to the positive terminal of the full-wave rectifier 2 drives the electromagnetic coil 4. One terminal of the electromagnetic coil 4 is coupled to the collector of the transistor 3, while the other terminal thereof is grounded. The electromagnetic coil 4 is energized and de-energized by turning on and off the transistor 3. The electromagnetic coil 4 is de-energized to trip the circuit breaker (not shown) when the controlled voltage V.sub.0 becomes less than the predetermined level. The circuit breaker associated with the electromagnetic coil 4 can be made only when the electromagnetic coil 4 is energized. The circuit breaker cannot be made when the electromagnetic coil 4 is de-energized.
The gate of an N-gate thyristor 5 coupled in series with a resistor R.sub.1 across the full-wave rectifier 2 is controlled by a voltage at a middle point A of the voltage divider consisting of a resistor R.sub.2 and a variable resistor R.sub.3. A Zener diode 6 is coupled across the N-gate thyristor 5 in opposite polarity. The gate of a P-gate thyristor 7 coupled in series with a resistor R.sub.4 across the full-wave rectifier 2 is controlled through a Zener diode 9 by the voltage at the middle point B of the voltage divider consisting of a resistor R.sub.5 and a variable resistor R.sub.6. Further, a capacitor 8 is coupled across the anodes of the N-gate thyristor 5 and P-gate thyristor 7. A resistor R.sub.7 connects the anode of the P-gate thyristor 7 and the point B coupled to the base of the transistor 3.
The operation of the circuit of FIG. 3 is as follows. Before the voltage is supplied from the controlled power source 1, the P-gate thyristor 7 is turned off. Thus, so long as the controlled voltage V.sub.0 is low, the base voltage of the transistor 3 remains at the high level, such that the transistor 3 remains turned off. The electromagnetic coil 4 is thus de-energized and the circuit breaker is tripped and is incapable of being made.
When the controlled power source 1 is normal and the controlled voltage V.sub.0 rises above a first minimum excitation voltage level, the voltage at the divider point B rises above the Zener voltage of the Zener diode 9 to turn on the Zener diode 9, thereby turning on the P-gate thyristor 7. The base voltage of the transistor 3 is thus reduced through the resistor R.sub.7, and the transistor 3 is turned on to energize the electromagnetic coil 4. The circuit breaker associated with the electromagnetic coil 4 thus becomes capable of being made. The first minimum excitation voltage level for turning on the electromagnetic coil 4 can be set by adjusting the voltage at the divider point B by means of the variable resistor R.sub.6.
When the P-gate thyristor 7 is turned on, the N-gate thyristor 5 is forcibly turned off by the commutation current of the capacitor 8. The P-gate thyristor 7 remains turned off until the controlled voltage V.sub.0 falls below a second minimum excitation voltage level. When the controlled voltage V.sub.0 falls below the second minimum excitation voltage level, the voltage at the divider point A coupled to the gate of the N-gate thyristor 5 becomes less than the voltage of the Zener diode 6, namely, the voltage at the anode of the N-gate thyristor 5. As a result, the N-gate thyristor 5 is turned on, and the commutation current from the capacitor 8 turns off the P-gate thyristor The transistor 3 is thus turned off and the electromagnetic coil 4 is de-energized, such that the circuit breaker is tripped and becomes incapable of being made.
By the way, the second minimum excitation voltage level for triggering the tripping operation in the case of abnormally low voltage is set lower than the first minimum excitation voltage level such that the circuit exhibits hysteresis and the operation is stabilized. When the controlled voltage V.sub.0 rises above the first minimum excitation voltage level again, the electromagnetic coil 4 is again energized as described above.
The above trip control device for a circuit breaker, however, has the following disadvantage. The circuit breaker is tripped (i.e., rendered incapable of being made) only when an abnormal low level of the controlled voltage V.sub.0 is detected. Thus, in the case where an overvoltage exceeding the maximum allowable level is applied to the circuit breaker due to the misconnection of the controlled power source 1, etc., the circuit breaker is not tripped and remains capable of being made. As a result, depending on the load connected to the circuit breaker, the destruction of expensive equipment or a grave failure accident such as fire may ensue.